Bragg gratings are conventionally employed as filters in optical communication systems. The gratings are formed with regions of differing (i.e., high and low amplitude) refractive index in the direction of light propagation and they reflect light over a spectral range .delta..lambda..sub.B centred on a wavelength .lambda..sub.B =2nP where
.lambda..sub.B =Bragg (centre) wavelength, PA1 n=effective (average) refractive index, and PA1 P=grating period.
The spectral region or bandwidth .delta..lambda..sub.B over which reflection occurs depends upon the strength of the grating (i.e., the amplitude or depth of refractive index modulation) and the length of the grating. In the case of a 10 cm long grating that is formed to provide a centre wavelength .lambda..sub.B of 1.55 .mu.m, the reflection bandwidth would be in the order of 0.01 nm.
The above described Bragg gratings are employed conventionally as optical filters which, in being formed within optical fibres, exhibit low insertion loss, and as transducers in strain or temperature responsive sensor devices. Also the gratings when chirped are employed as dispersion compensators and function as such to reflect different spectral components of light from different axially spaced positions along the length of the grating.
Chirping has the effect of expanding the width of the reflection peak and is achieved by varying the periodicity of the grating in the direction (Z) of light propagation. The chirping may be linear in which the grating period P=P.sub.O +.alpha.Z or non-linear in which case the grating period P=P.sub.O +.alpha.Z.sup.n for example.
One problem that is inherent in the above described gratings, including those that are chirped to exhibit an expanded spectral width, is that they cannot be tuned to a significant extent. However, it would be useful if a broadband grating of the described type could be produced for use in processing a single channel signal to facilitate a reduction in the degree of tuning required to match a source to the grating. This in turn would facilitate replacement of one source with another without there being a need for critical component matching. Another problem that is inherent in the known gratings is that they cannot be employed singly to perform filtering, dispersion compensation or other functions simultaneously on multiple channels at different wavelengths. In this respect it has been recognised by the inventors that it would be useful if a single grating could be employed to perform equivalent functions on multiple communication channels such as exist in wavelength division multiplexing (WDM) systems.